High-voltage high-power converters have always been a key technology for the application of power electronics in power systems and high-power electrical drive. A switch series connection technology or a multilevel technology must be employed in the case that the voltage required in practical applications exceeds the withstanding voltage of a single power semiconductor device. The withstanding voltage of conventional high-voltage power semiconductor devices is approximately 1-5 kV, and among them, those ordinary devices IGBT that are commonly used only have a withstanding voltage of 1200V approximately. Thus, use of a device having a withstanding voltage of 3400V could lead to much higher cost than use of those conventional high-voltage power semiconductor devices; even if a device having a higher withstanding voltage is used regardless of its cost, the high-voltage operating requirements of a power system cannot still be met without using the switch series connection technology or the multilevel technology. On the other hand, continuous improvement of the withstanding voltage level of these devices makes it possible that switching frequency becomes lower and lower, thereby increasing the volume and weight of a converter system.
For a high-voltage converter circuit, direct series connection of devices is the last resort. This scheme indeed brings the advantage of a relatively simple structure, but an extremely high change rate in switching voltage could still lead to the problems in the aspect of electromagnetic compatibility, and also degrade the reliability of load equipment and shorten the service life thereof. Furthermore, the voltage sharing control method for devices will be more difficult as the number of series connections rises, and requires a larger withstanding voltage margin, therefore, it can be concluded that the switch series connection technology is unsuitable for independent use in the power system.
As a result, it stands to reason that a multilevel circuit is used in the converter. The multilevel circuit can be applied to DC/AC, DC/DC, AC/DC and AC/AC, and for ease of description, illustration is mainly made below from an inversion (i.e. DC/AC) view.
(1) Power Switch
Reverse conducting switch is commonly used in a voltage source converter, and it may be composed of two independent devices: power semiconductor switch and antiparallel power diode and may also be an integrated device; for simplicity, it is herein referred to as switch (K, its symbol is shown in the circuit of FIG. 3), and the positive and negative poles of the switch are in directions that are just opposite to the polarities of the antiparallel power diode. K that is commonly used includes insulated gate bipolar transistor (IGBT) and power metal-oxide-semiconductor field effect transistor (Power MOSFET) device with the antiparallel power diode, and may also be thyristor, integrated gate commutated thyristor (IGCT), junction field effect transistor (Power JFET), and other novel devices like various types of silicon carbide power switches. In the circuit of FIG. 10, Power MOSFET is applied in K. A combined switch, which is formed by series connection of a plurality of reverse conducting switches, can still be perceived as a switch in the present invention.
(2) Several Important Multilevel Converter Circuits that Exist at Present
The first circuit: diode clamped multilevel circuit, which was first seen in the IEEE IAS conference paper (A. Naba) in 1980;
The second circuit: flying capacitor damped multilevel circuit, which was first seen in the IEEE PESC annual meeting paper (T. A. Meynard) in 1992;
The third circuit: unified clamped multilevel circuit, which was first seen in the IEEE IAS conference paper (F. Z. Peng) in 2000;
The fourth circuit: cascade multilevel circuit, which was first seen in the PESC conference paper (M. Marcheson) in 1988.
The first and second circuits have the major problem that: the complexity of these circuits is rapidly raised due to increase of the level number, the number of components and devices times fast (the former is switch devices and clamped diodes, and the latter is clamped capacitors); more seriously, impact from distributed inductance and control difficulty are also increased remarkably; in fact, applications of seven levels or above are rarely seen.
The third circuit has the major problem that increase of the level number results in faster rise of the number of components and devices in this circuit than the previous two circuits, and this circuit has not been put into practical application in industry yet. As a matter of fact, the third circuit is only theoretically meaningful, and the previous two circuits are special cases of the third circuit, respectively.
The foregoing shortcomings in the first, second and third circuits are not found in the fourth circuit; this circuit is capable of voltage balancing by means of an independent power source, is easy to realize modularization (H-bridge is served as unit module) and has been widely applied in medium-voltage variable frequency conversion, and its alternating current voltage is within 10 kV in general. Typically, a set of independent power source needs to be provided for every unit in the fourth circuit, so a quite complex structure of the main transformer of the apparatus is caused, which further places a limitation upon further rise of the level number.
In the field of reactive applications (e.g. one of the flexible power transmission devices for power system: STATCOM), the fourth circuit is free from the limitation of multiple paths of independent power sources, however, with the increase of the level number, there are still great challenges in voltage sharing problems.
(3) The Fifth Circuit, Balance Cascade Multilevel Converter
It is also known as “self-balance cascade multilevel”, its capability of realizing automatic voltage sharing of the converter units is the most prominent feature of the new circuit and was made public in the doctoral thesis of Zhejiang University (F. Zhang) 2006, and in fact, this circuit is a variant of the third circuit. However, this circuit also has a few problematic issues: low-voltage power supply and high-voltage output, so it is unsuitable for common high-voltage applications; energy needs to be transferred among units many times, making efficiency a great problem; during balance actions, there is no restriction mechanism for balancing current impact; and all circuit elements needs to be closely connected as a whole, causing a large difficulty in achieving modular combined manufacturing.
(4) The Sixth Circuit, Modular Multilevel Converter (MMC)
This circuit was first seen in the IEEE PowerTech Conference paper (A. Lesnicar and R. Marquardt) in 2003 The number of devices required in this circuit is linearly proportional to the level number, and this circuit is also suitable for modular manufacturing and particularly for ultrahigh-voltage applications (e.g. HVDC Light) in power system, however, voltage sharing control for its modules is still quite problematic, so its practical applications are rarely seen.